Heat-labile regulatory factor is required for 3' processing of histone precursor mRNAs.

In addition to Sm antigen-type small nuclear ribonucleoprotein particle(s) [snRNP(s)], at least one more factor is involved in the in vitro 3' processing of histone precursor mRNAs (pre-mRNAs) in a HeLa cell nuclear extract. This factor can be completely inactivated by mild heat treatment but is resistant to digestion by micrococcal nuclease and is not immunoprecipitated by antisera of the Sm serotype. Both snRNP (the presumed human homologue of the U7 snRNP of the sea urchin) and the heat-labile factor described above show closely similar properties when fractionated on DEAE, heparin, and Mono Q columns. Fractions, after extensive purification, still contain both heat-labile factor and snRNP activity. When analyzed by gel filtration, the heat-labile component distributes bimodally, the smaller component possessing an apparent molecular weight on the order of 40,000, and the larger, of ca. 300,000.